In order to identify by immunologic techniques structural features associated with the calmodulin-dependent regulation of myosin function, we have prepared hybridoma antibodies directed against purified smooth muscle myosin or its proteolytic fragments as well as against the purified myosin light chain kinase. These antibodies will be used as probes of myosin or myosin light chain kinase regulation and activity, to map specific functional domains of the proteins, and to study myosin and myosin light chain kinase expression during differentiation of smooth muscle and non-muscle cells.